The aim of this proposal is to measure the ability of cats to discriminate differences among spectrally complex acoustic stimuli (i.e., vowels and narrow-band approximation of single-formant vowels). Psychoacoustic studies will be conducted in behaviorally trained cats to measure their discrimination thresholds for changes in these complex stimuli in both quiet and continuous noise. 1) We will measure vowel discriminations among vowels with slightly different formant structures, especially at high sound levels. 2) We will measure vowel discriminations in continuous background noise to determine whether discrimination differences exist in loud background noise. 3) We will measure vowel discrimination thresholds, both in quiet and noise, following surgical sectioning of the crossed olivocochlear bundle (COCB) to determine the contribution of olivocochlear efferents in discriminating complex stimuli at high sound levels and at high noise levels. 4) We will measure discrimination thresholds for narrow-band approximations to single-formant stimuli in quiet and noise, and redetermine these thresholds following surgical sectioning of the COCB. 5) We will measure discrimination thresholds for modulation frequency for these narrow-band, single-formant stimuli as well as determine the discriminability of modulation frequency as a function of sound level. Results of psychoacoustic experiments on the discriminability of complex stimuli will be related to models of auditory system functioning based on physiological data obtained in cats under similar stimulus presentation conditions. These studies will provide important psychophysical data on complex stimulus discrimination in a species widely used in physiological studies of auditory system function.